The present invention relates to hydraulic control systems and more particularly relates to such system for controlling pitch and tilt movement of the working blades of bulldozers, or the like.
It is known to provide a tractor with a pair of fore-and-aft extending push arms having their rear ends pivotally connected to opposite sides of the tractor frame and hving their forward ends pivotally connected to a dozer blade such as to define a transverse axis about which the blade may be selectively adjusted by means of a pair of extensible and retractable hydraulic actuators respectively mounted between the blade and the pair of push arms. Such a push arm and blade structure is disclosed in U.S. Pat. 2,817,168 issued to Mullin on Dec. 24, 1957.
The Mullin patent describes a control system which includes a separate valve for controlling each of the pair of actuators and a separate lever for controlling each of the pair of valves. Thus, both valves must be simultaneously actuated in the same direction to cause simultaneous retraction or extension of the actuators so as to effect a pitch movement of the blade. This operation is not entirely satisfactory since it requires a good deal of operator skill to operate the levers connected to the valves so as to cause equal displacement of the actuators so as to effect a pitch movement of the blade without also effecting a tilt movement of the blade.
The aforementioned problem of unequal actuator displacement during pitch adjustment of the blade is overcome by the structure disclosed in U.S. Pat. No. 3,184,869 issued to Olsen on May 25, 1965. As disclosed in the Olsen patent, the control system includes a single pitch control valve for both actuators, the latter being connected in parallel with each other. A 50/50 rotary flow divider is connected between the valve and the actuators for ensuring equal flow to and from the actuators when the pitch control valve is actuated. For the purpose of causing tilt adjustments of the blade, a reversing valve is placed between the flow divider and one of the actuators and operable such as to cause one actuator to extend or to retract respectively when the pitch control valve is operated to cause the other actuator to retract or extend. This system is not entirely satisfactory since it requires the operator to shift the reversing valve in order to change the system between blade-tilt and blade-pitch modes.